Summary Although alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD) are highly comorbid, little is known about the neural substrates that contribute to this comorbidity. Moreover, despite the frequent co- occurrence of AUD and PTSD, there is marked variability in the likelihood of developing these disorders and even less is known about the neural substrates that might confer vulnerability or resilience to AUD and PTSD. The scientific premise of this application is that a better understanding of the circuitry and neurobiology that differentiate these vulnerable and resilient populations may reveal novel targets for the development of more effective treatments for individuals suffering from the comorbid condition. To address this challenge, we have extensively characterized a rodent model of early life stress, adolescent social isolation (aSI), and have generated encouraging support for the face, construct and predictive validity of aSI as a model of vulnerability to AUD and PTSD. For example, relative to rats group-housed throughout adolescence (aGH), aSI rats exhibit many behaviors linked with heightened risk of AUD and PTSD, including deficits in fear extinction and enduring increases in ethanol drinking behaviors. Notably, a relatively mild fear conditioning procedure significantly increased ethanol self-administration in aSI rats for at least 8 weeks while having no effect on ethanol drinking in aGH subjects. Published and ongoing neurobiological studies have identified profound adaptations that may contribute to these behavioral phenotypes, including increased measures of excitability within the basolateral amygdala (BLA) and ventral hippocampus (vHC), two highly interconnected brain regions that play an integral role in many of the emotional behaviors that are disrupted in AUD and PTSD. Based on these findings, the studies outlined in this application will employ a multidisciplinary experimental design to determine if the fear conditioning procedure leads to the expression of core behavioral symptoms of AUD and PTSD in aSI rats and whether aGH subjects will be resilient to this stressor. Neurobiological studies will determine if the fear conditioning procedure exacerbates aSI-associated neurobiological adaptations in the BLA and vHC, and specifically within the BLA-vHC circuit, and whether a strengthening of this circuit plays a causal role in AUD/PTSD-like behavioral phenotypes promoted by this model. Additional studies will test a novel therapeutic strategy to reverse the maladaptive behaviors promoted by aSI + fear conditioning. Based on other emerging findings, these studies will also test the innovative hypothesis that aSI + fear conditioning promotes similar neural adaptations in male and female rats but that the behavioral phenotypes engendered by these stressors may be sexually dimorphic. Collectively, these studies will further strengthen the validity of aSI as a model of heightened vulnerability to comorbid AUD and PTSD and potentially lead to the identification of novel neurobiological targets for the development of much needed treatments for the comorbid condition.